Prosthetic cardiac valves have been used for many years to treat cardiac valvular disorders. The native heart valves (such as the aortic, pulmonary and mitral valves) serve critical functions in assuring the forward flow of an adequate supply of blood through the cardiovascular system. These heart valves can be rendered less effective by congenital, inflammatory or infectious conditions. Such damage to the valves can result in serious cardiovascular compromise and even death. For many years, the definitive treatment for such disorders was the surgical repair or replacement of the valve during open heart surgery, but such surgeries are prone to many complications. More recently, a transvascular technique has been developed for introducing and implanting a prosthetic heart valve using a flexible catheter in a manner that is less invasive than open heart surgery.
In this technique, a prosthetic heart valve is mounted in a crimped state on the end portion of a flexible catheter and advanced through a blood vessel of the patient until the valve reaches the implantation site. The valve at the catheter tip is then expanded to its functional size at the site of the defective native valve such as by inflating a balloon on which the valve is mounted.
FIG. 1 shows a known percutaneous heart valve 10 in its deployed or expanded state. The valve 10 comprises a flexible prosthetic valve member 12 attached to an expandable frame, or support stent, 14 with sutures 16. The frame 14 includes angularly-spaced, axial struts 18 and circumferentially extending, zig-zag struts 20 secured to the axial struts 18. Between each pair of axial struts 18, each strut 20 comprises two linear strut members 22a, 22b forming a bend in the strut to facilitate crimping of the valve 10 to a smaller diameter for percutaneous delivery of the valve. As can be appreciated, the easiest and most straightforward way of attaching the valve member 12 to the frame 14 is when both the frame 14 and the valve member 12 are in the expanded state shown in FIG. 1. The assembled valve 10 typically is stored in the expanded state or a partially crimped state and then fully crimped to a much smaller profile in the operating room just prior to implantation.
An important characteristic of a percutaneous prosthetic heart valve is its ability to be crimped to as small diameter as possible to permit the crimped valve to be advanced through the blood vessels to an implantation site. Another important characteristic of a percutaneous heart valve is its ability to retain an expanded shape once implanted. To maximize circumferential and radial rigidity of the valve frame, and therefore enhance the ability of the frame to retain an expanded shape once implanted, it is desirable to maximize the angle θ between strut members 22a, 22b. Ideally, the struts 20 should be nearly circular (i.e., the angles θ are slightly less than 180 degrees) to provide maximum rigidity. Moreover, by increasing the rigidity of the struts, less metal can be used for forming the frame, which allows the valve to be crimped to a smaller profile.
Unfortunately, forming the struts 20 with angles θ that are greater than 90 degrees can lead to uneven and unpredictable crimping. Thus, if the valve assembly is assembled in its expanded, functional shape, then in order to permit even and predictable crimping of the frame to a predetermined profile suitable for percutaneous delivery, rigid struts with obtuse angles θ cannot be utilized.